/* define exit codes */int NORMAL= 0;int TIMEOUT= 1;int follow_edge_to_wall()  {    long timeout= mseconds() + 4000L;  while (1)  {    if (left_eye() == 0) veer_left();    else if (right_eye() == 1) veer_right();    else forward();    if (left_touch() || right_touch()) return NORMAL;    if (mseconds() > timeout) return TIMEOUT;  }}